Update to /users PUT

@codal-sara These changes are made based on the comments from: https://github.com/DoSomething/user-api/pull/5

* Updating a user requires a PUT to `/users/:_id` where `:_id` is the unique document identifier for a user. Gone are the query parameters specifying a user by either email, drupal uid, or mobile number - though only for this endpoint.

* Places where `doc_id` was used have been changed to simply `_id`.